Ensure that timestamp is always a Date object

lib/www/client/source/src/views/Plan.vue

@@ -440,6 +440,7 @@ export default {
       function isDay (srss, ts, lat, lng) {
         if (isNaN(srss.sunriseEnd) || isNaN(srss.sunsetStart)) {
           // Between March and September
+          ts = new Date(ts);
           if (ts.getMonth() >= 2 && ts.getMonth() <= 8) {
             // Polar day in the Northern hemisphere, night in the South
             return lat > 0;